Tactile Warning Surface (TWS) products are required in certain locations under the Americans with Disabilities Act Accessibility Guidelines (ADAAG). The ADAAG defines certain types of applications, including curb ramps/pedestrian crossings, commercial applications (e.g., big box retailers, hotels and restaurants) and transit facilities (e.g., commuter rail, rapid transit and Bus Rapid Transit (BRT)). The visually impaired may elect to utilize TWS products to detect hazardous drop-offs (platform edge/loading dock) and hazardous vehicular areas (curb ramps on street corners and intersections, uncurbed transition between pedestrian and vehicular areas such as at the front of big box retail establishments).
Visually impaired and fully sighted persons may rely on a combination of visual cues (color contrast), tactile cues (sweeping cane, sole of shoe, through wheelchair wheels, walker wheels), and audio cues (sound attenuation, which can be achieved by use of dissimilar materials such as composite TWS and concrete substrate) when electing to use TWS products as a means of edge and hazardous vehicular area detection.
TWS products define a series of spaced raised truncated domes. See, e.g., U.S. Pat. No. 7,001,103 for a discussion of TWS products. These products are typically installed in curb ramps, pedestrian ways and commercial areas by setting into the fresh concrete a plastic, composite or metal TWS product that defines on its upper surface the series of spaced raised truncated domes required by the ADAAG. Although such Cast-In-Place (CIP) TWS products are easy to install into wet concrete (typically taking only a few minutes), replacement is difficult and time consuming, and replacement costs are high, because the underlying substrate must be at least partially destroyed in order to remove an installed product, and then reconstructed for the replacement product.
Some of these CIP TWS Units define a relatively thin upper surface layer supported underneath by spaced honeycomb-like lower walls that are set in fresh concrete. Air can be trapped between the lower walls, which creates areas underneath the CIP TWS Unit that are not supported by the underlying substrate. Because they are thin to begin with, and in spots not supported, these CIP TWS Units can fatigue and crack under moderate or heavy loading, such as can be caused by pallet jacks, fork lifts and vehicles, for example. Also, due to the plurality of intersecting lower walls that are embedded in concrete, in some cases these CIP TWS Units cannot be replaced without tearing up and then rebuilding the concrete structure in which they were set; this is a time consuming and expensive proposition.
Another issue with ADAAG-compliant TWS products is that the projecting domes can be broken or sheared off by snowplows or the like, requiring replacement. Some fiberglass-reinforced epoxy resin TWS products have a body that is reinforced by a woven fiberglass mat. However, the domes are constructed of pure resin without any fiberglass reinforcement for impact resistance. These TWS products thus have projecting domes that are inherently weaker than the body. The domes thus can be more easily cracked, broken or sheared off.
Some CIP TWS Units are set into fresh concrete with fasteners that pass through holes located in the domes. There are also CIP TWS Units in which the head of the fastener is shaped like a dome, in which case the fastener is located in place of one of the domes. In both such cases, if a dome is sheared or broken off, there is danger that the head of the fastener can be sheared or broken off, or at a minimum the fastener can be loosened. If this happens, the TWS product can come loose and present a tripping hazard.
The prior state of the art for new construction includes composite shell CIP TWS Units. Composite shell CIP TWS Units are quickly and economically installed; however, if the installer is not diligent CIP TWS Units are susceptible to air entrapment underneath the CIP TWS Unit and are thus susceptible to fatigue and cracking failure due to repetitive and/or heavy loading. Fatigue and cracking failure under repetitive heavy loading may also occur along the relatively thin perimeter flange structure. Once installed, CIP TWS Units are permanently embedded into the concrete substrate and it is thus difficult, invasive, time consuming, and costly to remove and replace CIP TWS Units when maintenance is required.
Another solution is a surface applied (SA) TWS panel that is applied to a finished substrate. A SA TWS panel is typically mechanically fastened (e.g., with a nylon sleeve anchor with a stainless steel pin) and adhered (e.g., using single component urethane adhesive) to the underlying substrate, and then caulked around the perimeter to compensate for substrate irregularities, minimize water intrusion, and provide a superior architectural finish. Installation takes 10-15 minutes for a 2′×4′ SA TWS panel. Replacement of a SA TWS panel is easier than with a CIP TWS Unit, and is typically accomplished by removing the fasteners, heating the SA TWS panel to break the adhesive bond with the underlying substrate, prying the TWS panel off the substrate, removing existing adhesive, and installing a new SA TWS panel. The substrate basically remains intact. Perhaps 1 to 1½ hours labor is involved. Replacement cost is thus moderate. However, these SA TWS panels can more easily loosen or dislodge as compared to CIP TWS units. For example, a protruding edge or corner of the SA TWS panel can be caught by a snow plow and lifted. This can present a safety hazard. SA TWS panels are thus not as acceptable as CIP TWS Units. SA TWS panels are an ideal solution for retrofit applications; CIP or replaceable (REP) TWS Units are an ideal, quick, and economical solution for new construction. The elevation of the body of a SA TWS panel is at least ⅛″ above the surface of the underlying substrate; consequently, the body of the SA TWS panel is potentially vulnerable to damage from snow removal operations. The body of CIP or REP TWS Units are flush mounted relative to the adjacent substrate; consequently, the body of the TWS Unit is shielded or protected from damage due to snow removal operations. Flush mounted TWS Product installations offer superior performance when compared to surface mounted TWS Product installations. As the fasteners in SA TWS Panels are located within the truncated dome, they are vulnerable to damage from snow removal or similar shearing type action that the domes may be subjected to under everyday use.
There is thus the need for a wet-set replaceable TWS Unit that is rugged so less likely to be damaged or loosened from the underlying substrate, uses fasteners that are less likely to be damaged by heavy equipment, and can be quickly, easily and economically replaced without reconstruction of the underlying substrate (such as a curb ramp or transit platform), or dealing with adhesive application and removal.